A network of balloon-borne radiosonde observations employing
chilled-mirror hygrometers for water and electrochemical
concentration cells for ozone has been operated since the late 1990s in
the Tropical Pacific to capture the evolution of dehydration
of air parcels advected quasi-horizontally in the Tropical Tropopause
Layer (TTL). The analysis of this dataset is made on isentropes
taking advantage of the conservative properties of tracers moving
adiabatically. The existence of ice particles is diagnosed by
lidars simultaneously operated with sonde flights. Characteristics
of the TTL dehydration are presented on the basis of individual
soundings and statistical features. Supersaturations close to
80% in relative humidity with respect to ice
(RH<sub>ice</sub>) have been observed in subvisible cirrus clouds
located near the cold point tropopause at extremely low temperatures
around 180 K.
Although further observational evidence is needed to
confirm the credibility of such high values of RH<sub>ice</sub>,
the evolution of TTL dehydration is evident from the data in isentropic scatter
plots between the sonde-observed mixing ratio (OMR) and the minimum
saturation mixing ratio (SMR<sub>min</sub>) along the back
trajectories associated with the observed air mass.
Supersaturation exceeding the critical value of homogeneous ice
nucleation (OMR > 1.6 &times; SMR<sub>min</sub>) is frequently
observed on the 360 and 365 K surfaces indicating that cold
trap dehydration is in progress in the TTL. The near
correspondence between the two (OMR ~ SMR<sub>min</sub>) at
380 K on the other hand implies that this surface is not
sufficiently cold for the advected air parcels to be dehydrated.
Above 380 K, cold trap dehydration would scarcely
function while some moistening occurs before the air parcels
reach the lowermost stratosphere at around 400 K where OMR
is generally smaller than SMR</sub>min</sub>.